The relatively high strength and light weight of fiber-reinforced composite materials (e.g., materials comprised of fibers of high strength and modulus, such as carbon fibers, glass fibers and the like which are embedded in or bonded to a resin matrix) have attracted interest in the aircraft industry as a means to construct aircraft components, for example, primary flight surfaces such as aircraft wings, horizontal stabilizers and the like. The assembly of at least two composite parts wherein their laminate principal directions form a non-singular angle brings about a series of design and manufacturing inconveniences. In extreme situations, such design and/or manufacturing inconveniences compromise the applicability of composite materials for particular components even though the physical characteristics of composite materials are more desirable as compared to components formed of light weight metal (e.g., aluminum).
Best weight savings and traditional structural behavior of wings and stabilizers are obtained when their composite skin layer angles are defined relative to their main load carrying directions, usually aligned with the longest direction of these parts which is typically the principal laminate direction (sometimes also referred as the laminate reference axis).
The junction zones between two or more regions of a part with conflicting laminate principal directions (as may be expected on junctions between aircraft wings and stabilizer skin sides) is usually designed by manufacturing such parts separately, and then joining the parts later by mechanical means, such as metal splices. However such a splicing technique is disadvantageous for a number of reasons. For example, splicing may cause the appearance of stress concentration points and fiber interruptions due to the need for joining rivets and associated drillings, thereby highly reducing the composite part's strength. Also, the use of rivets involves a time consuming additional manufacturing step.
If carbon fibers are the reinforcing media in the composite material, then galvanic interaction may occur between the carbon fibers of the composite material and aluminum of the splices. Such galvanic interaction will thereby cause corrosion of the metal forming the splices. As a consequence, the carbon fibers must be isolated physically from the metal splices using adjacent adhesive layers and/or glass fiber-reinforced composite layers.
Thermal expansion loads also need to be accounted for when using metal splices due to the different thermal expansion coefficients of the materials (i.e., since metals such as aluminum, expand and contract more than composite materials formed, for example, of carbon reinforcing fibers).
There is also the problem of increased assembly tolerances thereby requiring the use of liquid and/or rigid shims. Generally speaking therefore an increased number of parts, higher production costs and periodic maintenance issues need to be resolved when conventional metal splices are used to join composite material parts.
Another known technique that is employed to assemble wings or stabilizer skins is positioning alternating layers from the two principal lamination axes among each other in the junction zone between the parts. This technique also poses several problems including:                all loads are transmitted through interlaminar shear, aggravated by involving regions with different elastic properties;        potential unbalanced laminates at the junction zone;        the structural understanding of these joints are normally associated with complex numerical analyses, many times validated only by expensive physical tests; and/or        a concentrated superposition of plies may cause assembly problems with spars, stringers or ribs.        
Recently, in U.S. Pat. No. 6,908,526 issued on Jun. 21, 2005 (the entire content of which is expressly incorporated herein by reference), a process for manufacturing part or all the composite material assembly comprising at least two zones in which the principal directions form an angle different than 0° and 180°, making use of a continuous lay-up technique. Although the possibility to perform a continuous lay-up for some lamination techniques is possible, the process proposed by the US '526 patent has a distinct disadvantage of providing at the same ply level different lay-up directions, namely a 0° layer on one side of the lay-up is reflected in the other side as a ±α° layer.
It would therefore be desirable if balanced laminate structures could be formed cost-effectively and efficiently. It is towards fulfilling such needs that the present technology is directed.
Broadly, the present technology is embodied illustratively in methods and resulting laminate structures wherein the lay-up of composite materials is accomplished more symmetrically and more continuously as compared to prior techniques to form a composite structure from two composite parts in which their principal laminate directions form a non-singular angle.
According to one exemplary implementation, a method for making a composite structure comprised of at least two parts joined to one another at a central junction zone with a non-singular angle, by laying up 0° composite material plies relative to a central coordinate system COORD C of the central junction zone so as to achieve +(90°−θ°/2) plies relative to left and right coordinate systems COORD L and COORD R, respectively, of the at least two parts, and laying up +(180°−θ°) and −(180°−θ°) plies relative to the central coordinate system COORD C of the central junction zone so as to achieve −(90°−θ°/2) plies relative to left and right coordinate systems COORD L and COORD R, respectively, of the at least two parts. In preferred forms, the lay-up steps are practiced sequentially.
According to another exemplary implementation, the method may comprise laying up 90° composite material plies relative to the central coordinate system COORD C of the central junction zone so as to achieve −θ°/2 plies relative to the left and right coordinate systems COORD L and COORD R, respectively, of the at least two parts; and laying up +(90°−θ°) and −(90°−θ°) composite material plies relative to the central coordinate system COORD C of the central junction zone so as to achieve +θ°/2 plies relative to the left and right coordinate systems COORD L and COORD R, respectively, of the at least two parts.
In especially preferred embodiments, the lay-up steps are practiced sequentially one after the other. The nonsingular angle θ may be greater than 90° and less than 180°. Thus, for example, θ may be between about 120° or about 135°.
An exemplary composite structure may be comprised of a central junction zone, and at least two parts joined to one another at the central junction zone with a non-singular angle, the composite structure including (i) 0° composite material plies relative to a central coordinate system COORD C of the central junction zone, the 0° composite material plies forming +(90°−θ°/2) plies relative to left and right coordinate systems COORD L and COORD R, respectively, of the at least two parts, and (ii) +(180°−θ°) and −(180°−θ°) plies relative to the central coordinate system COORD C of the central junction zone, the +(180°−θ°) and −(180°−θ°) plies forming −(90°−θ°/2) plies relative to left and right coordinate systems COORD L and COORD R, respectively, of the at least two parts.
In other implementations, the composite structure may additionally or alternatively comprise (iii) 90° composite material plies relative to the central coordinate system COORD C of the central junction zone, the 90° plies forming θ°/2 plies relative to the left and right coordinate systems COORD L and COORD R, respectively, of the at least two parts; and (iv) +(90°−θ°) and −(90°−θ°) composite material plies relative to the central coordinate system COORD C of the central junction zone, the +(90°−θ°) and −(90°−θ°) plies forming +θ°/2 plies relative to the left and right coordinate systems COORD L and COORD R, respectively, of the at least two parts.
The methods and composite structures may be in the form of an aircraft component, for example, primary flight structures such as an aircraft wing, horizontal stabilizer or the like.